Process for fabricating multilayer printed circuits

ABSTRACT

For attaining adhesion between copper circuitry innerlayers and pre-preg layers in a multilayer printed circuit, a conversion coating of copper oxide is provided on the metallic copper surfaces in a manner which develops an altered topography of the underlying metallic copper surfaces. Thereafter, a controlled dissolution and removal of a substantial amount of the copper oxide is effected, in a manner which does not adversely affect the already-developed topography of the underlying metallic copper, and such that the innerlayer, at the time of assembly with the pre-preg layers, has copper surfaces consisting of the topographically altered metallic copper and a relatively small amount of copper oxide thereon. Excellent bonding strengths are achieved with decreased incidence of pink ring formation as compared to conventional processes utilizing copper oxide for adhesion promotion. In addition, the copper ion containing solutions resulting from the controlled dissolution and removal of copper oxide can be economically utilized in formulating electroless copper plating baths.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of co-pending applicationSer. No. 07/783,974, filed Oct. 29, 1991, which in turn is acontinuation-in-part of application Ser. No. 07/734,247, filed Jul. 22,1991, now abandoned.

The present invention relates to printed circuits, and more particularlyto a process for fabricating a through-hole-containing multilayerprinted circuit.

Printed circuits containing one or more circuitry innerlayers are inprominent use today as demand increases for further and further weightand space conservation in electronic devices.

In the typical fabrication of a multilayer printed circuit, patternedcircuitry innerlayers are first prepared by a process in which a copperfoil-clad dielectric substrate material is patterned with resist in thepositive of the desired circuitry pattern, followed by etching away ofthe exposed copper. Upon removal of the resist, there remains thedesired copper circuitry pattern.

One or more circuitry innerlayers of any particular type or types ofcircuitry pattern, as well as circuitry innerlayers which mightconstitute ground planes and power planes, are assembled into amultilayer circuit by interposing one or more partially-cured dielectricsubstrate material layers (so-called "pre-preg" layers) between thecircuitry innerlayers to form a composite of alternating circuitryinnerlayers and dielectric substrate material. The composite is thensubjected to heat and pressure to cure the partially-cured substratematerial and achieve bonding of circuitry innerlayers thereto.

The so-cured composite will then have a number of through-holes drilledtherethrough, which are then metallized to provide a means forconductively interconnecting all circuitry layers. In the course of thethrough-hole metallizing process, desired circuitry patterns alsotypically will be formed on the outer-facing layers of the multilayercomposite.

It has long been known that the strength of the adhesive bond formedbetween the copper metal of the circuitry innerlayers and the curedpre-preg layers in contact therewith leaves something to be desired,with the result that the cured multilayer composite is susceptible todelamination in subsequent processing and/or use. In response to thisproblem, the art developed the technique of forming on the coppersurfaces of the circuitry inner-layers (before assembling them withpre-preg layers into a multilayer composite) a layer of copper oxide,such as by chemical oxidation of the copper surfaces. The earliestefforts in this regard (so-called "black oxide" adhesion promoters)produced somewhat minimal improvement in the bonding of the circuitryinnerlayers to the dielectric substrate layers in the final multilayercircuit, as compared to that obtained without copper oxide provision.Subsequent variations on the black oxide technique included methodswherein there is first produced a black oxide coating on the coppersurface, followed by post-treatment of the black oxide deposit with 15%sulfuric acid to produce a "red oxide" to serve as the adhesionpromoter, such as disclosed in A. G. Osborne, "An Alternate Route to RedOxide For Inner Layers", PC Fab. August, 1984, as well as variationsinvolving direct formation of red oxide adhesion promoter, with varyingdegrees of success being obtained. The most notable improvement in thisart is represented in U.S. Pat. Nos. 4,409,037 and 4,844,981 to Landauinvolving oxides formed from relatively high chlorite/relatively lowcaustic copper oxidizing compositions, and producing substantiallyimproved results in circuitry innerlayer adhesion.

As earlier noted, the assembled and cured multi-layer circuit compositeis provided with through-holes which then require metallization in orderto serve as a means for conductive interconnection of the circuitrylayers of the circuit. The metallizing of the through-holes involvessteps of resin desmearing of the hole surfaces, catalytic activation,electroless copper depositing, electrolytic copper depositing, and thelike. Many of these process steps involve the use of media, such asacids, which are capable of dissolving the copper oxide adhesionpromoter coating on the circuitry innerlayer portions exposed at or nearthe through hole. This localized dissolution of the copper oxide, whichis evidenced by formation around the through-hole of a pink ring or halo(owing to the pink color of the underlying copper metal therebyexposed), can in turn lead to localized delamination in the multilayercircuit.

The art is well aware of this "pink ring" phenomenon, and has expendedextensive effort is seeking to arrive at a multilayer printed circuitfabrication process which is not susceptible to such localizeddelamination. One suggested approach has been to provide the adhesionpromoting copper oxide as a thick coating so as to retard itsdissolution in subsequent processing simply by virtue of sheer volume ofcopper oxide present. This turns out to be essentiallycounter-productive, however, because the thicker oxide coating isinherently less effective as an adhesion promoter per se. Othersuggestions relating to optimization of the pressing/curing conditionsfor assembling the multilayer composite have met with only limitedsuccess.

Other approaches to this problem involve post-treatment of the copperoxide adhesion promoter coating prior to assembly of circuitryinnerlayers and pre-preg layers into a multilayer composite. Forexample, U.S. Pat. No. 4,775,444 to Cordani discloses a process in whichthe copper surfaces of the circuitry innerlayers are first provided witha copper oxide coating and then contacted with an aqueous chromic acidsolution before the circuitry innerlayers are incorporated into themulti-layer assembly. The treatment serves to stabilize and/or protectthe copper oxide coating from dissolution in the acidic mediaencountered in subsequent processing steps (e.g., through-holemetallization), thereby minimizing pink ring/delamination possibilities.

U.S. Pat. Nos. 4,642,161 to Akahoshi et al, 4,902,551 to Nakaso et al,and 4,981,560 to Kajihara et al, and a number of references citedtherein, relate to processes in which the copper surfaces of thecircuitry innerlayers, prior to incorporation of the circuitryinnerlayers into a multilayer circuit assembly, are first treated toprovide a surface coating of adhesion-promoting copper oxide. The copperoxide so formed is then reduced to metallic copper using particularreducing agents and conditions. As a consequence, the multilayerassembly employing such circuitry innerlayers will not evidence pinkring formation since there is no copper oxide present for localizeddissolution, and localized exposure of underlying copper, in subsequentthrough-hole processing. As with other techniques, however, processes ofthis type are suspect in terms of the adhesion attainable between thedielectric substrate layers and the metallic copper circuitryinnerlayers. This is particularly so in these reduction processes sincethe circuitry bonding surface not only is metallic copper, but alsopresents the metallic copper in distinct phases (i.e., (1)copper-from-reduction-of-copper oxide over (2) copper of the copperfoil) which are prone to separation/delamination along the phaseboundary.

U.S. Pat. Nos. 4,997,722 and 4,997,516 to Adler similarly involveformation of a copper oxide coating on the copper surfaces of circuitryinnerlayers, followed by treatment with a specialized reducing solutionto reduce the copper oxide to metallic copper. Certain portions of thecopper oxide apparently may not be reduced all the way to metalliccopper (being reduced instead to hydrous cuprous oxide or cuproushydroxide), and those species are thereafter dissolved away in anon-oxidizing acid which does not attack or dissolve the portionsalready reduced to metallic copper. As such, the multilayer assemblyemploying such circuitry innerlayers will not evidence pink ringformation since there is no copper oxide present for localizeddissolution, and localized exposure of underlying copper, in subsequentthrough-hole processing. Here again, however, problems can arise interms of the adhesion between the dielectric layers and metallic coppercircuitry innerlayers, firstly because the bonding surface is metalliccopper, and secondly because the metallic copper predominantly ispresent in distinct phases (i.e., (1) copper-from-reduction-of-copperoxide over (2) copper of the copper foil), a situation prone toseparation/delamination along the phase boundary.

Thus, notwithstanding recognition in the art of the need for astructurally sound multilayer composite, and art awareness of the causesof, and problems associated with, the pink ring phenomenon, there hasyet to be provided an economically feasible process which insuresfabrication of multilayer printed circuits which are not prone todelamination.

SUMMARY OF THE INVENTION

The present invention provides a process for the fabrication of amultilayer printed circuit containing metallized through-holes, in whichcopper circuitry innerlayers are interleaved with partially-cureddielectric substrate layers so as to assemble a multi-layer structurewhich is thereafter cured to provide a multilayer composite, and whichis thereafter treated to provide metallized through-holes and othernecessary features of a multilayer printed circuit.

In accordance with the invention, the circuitry innerlayers are uniquelyprocessed before utilizing them in assembly, with pre-preg layers, ofthe multilayer structure. Thus, the copper surfaces of the circuitryinnerlayers are first treated by conventional means to form a conversioncoating of copper oxide thereon in any thickness as might conventionallybe employed in the copper oxide adhesion promotion art. Thereafter, thecopper oxide conversion coating is treated with a dissolution agenteffective to substantially decrease the thickness of the copper oxidelayer by controllably dissolving and removing the copper oxide to asufficient extent such that--at the time when the circuitry inner-layeris arranged in contact with pre-preg layers to form the multilayerstructure--copper oxide is present on the copper surfaces of thecircuitry innerlayer, but in an amount which is less than thatoriginally provided and which, in any event, is only a relatively smallamount, such as an amount on the order of less than about 0.20 mg/cm²,more preferably less than about 0.12 mg/cm², still more preferably lessthan about 0.08 mg/cm², and more preferably at or below 0.05 mg/cm². Theso-processed circuitry innerlayers are then utilized in assembly of themultilayer structure, followed by curing, provision of through-holes,metallizing of through-holes, and the other conventional steps forproducing a finished multilayer printed circuit.

In the process of the present invention, there is a unique coaction ofeffects, although not entirely understood, leading to highlyadvantageous results. The initial formation of what is essentially aconventional copper oxide conversion coating on the copper surfacesleads to topographic alteration of the metallic copper surface. Thesubsequent substantial dissolution of the copper oxide coating iscarried out in a manner which essentially maintains thealready-developed topography of the metallic copper surface, and whichof course substantially lessens the amount of copper oxide on thosetopographically altered surfaces. When the multilayer structure isassembled, the innerlayer copper surfaces in contact with the pre-pregthus consist of this topographically-altered metallic copper surface andthe lessened amount of copper oxide thereover. In the heat and pressurecuring of the multilayer, the bonding of the innerlayer to the curedpre-preg is believed to involve, as the bonding surface of the circuitryinnerlayer, this topographically-altered metallic copper surface withthe further presence thereon of the lessened amount copper oxide. In thesubsequent processing of the multilayer, as in the formation andmetallization of through-holes, the incidence and/or extent of pink ringformation (more generally, attack of the oxide by processing media atany innerlayer area at which the oxide is exposed to the media) isdecreased as compared to conventional processes utilizing copper oxideas an adhesion promoter. This result is believed to be attained because,either as a consequence of the relatively small amount of oxide present,and/or the manner in which the oxide is arrived at, and/or otherfactors, the oxide present tends to resist dissolution in the variousprocessing media.

The mere decrease in the incidence or extent of pink ring formation inand of itself is not, of course, the key consideration, since thisresult would be readily achievable simply by use of clean copper bondingsurfaces with no oxide thereon (hence nothing present to dissolve in thesubsequent processing media and reveal any "underlying" pink coppermetal coloration). Rather, the ultimate consideration is to attain asoundly-bonded multi-layer structure at all areas of the structure, andthrough a bond which does not degrade in subsequent processing. Thus,decreasing "pink ring" as by utilizing untreated copper bonding surfacesis of no particular importance if the bond so achieved is notstructurally sound, i.e., if it is at the expense of the ultimatelydesired goal.

The key importance, then, of the present invention is that a soundlybonded multilayer structure is attained in a manner which does not makeprimary reliance on an oxide which would be prone to subsequentdissolution in through-hole processing media, as in conventionaltechnology. Rather, the innerlayer bonding surface is atopographically-altered metallic copper surface having only a relativelysmall amount of copper oxide thereon, and results in a very sound bond.At all areas of the structure where there is no opportunity forprocessing media to dissolve this small amount of copper oxide, theattained bond is of course retained to its high degree. At those areas(e.g., edges, through-hole surfaces) where subsequent processing mediacan potentially gain direct access to the surfaces, it would appearthat, for reasons not entirely understood, this relatively small amountof copper oxide tends to resist dissolution by the processing media, andhence no significant adverse alteration of the initially attained bondstrength occurs.

Thus, the present invention provides a means for eliminating thelocalized delamination which can occur at through-hole locations inprocesses employing conventionally formed copper oxide layers as anadhesion promoter for innerlayer circuits, but without at the same timecompromising on the strength, at all areas of the circuit, of thelamination bonding itself.

As elaborated upon hereinafter, and as set forth in the claims, thepresent invention provides an improvement in the fabrication ofmultilayer printed circuits. Thus, in a process for fabricating amultilayer printed circuit, in which: a multilayer structure isassembled as an alternating array of one or more copper circuitryinnerlayers and one or more layers of partially-cured dielectricsubstrate material; the multilayer structure is then subjected to heatand pressure to cure the structure into an integral multilayer compositein which copper surfaces of the innerlayer circuitry layers are bondedto the now-cured dielectric substrate material in contact therewith; andthe multilayer composite is then provided with one or more through-holeswhich are thereafter metallized; the improvement is characterized inthat the copper surfaces of the circuitry innerlayers, prior to assemblyin the multilayer structure, are processed to form thereon a conversioncoating of copper oxide, with the result of topographically altering theunderlying metallic copper surface; following which the copper oxidecoating is treated with a dissolution agent effective to controllablydissolve and remove the so-formed copper oxide, without significantalteration of the earlier generated topography of the underlyingmetallic copper surfaces, the dissolution and removal being to asufficient extent such that, when the circuitry innerlayer is arrangedin contact with the partially-cured dielectric substrate material,copper oxide exists on the copper surfaces but in an amount which isdecreased from that originally present and which, in any event, is arelatively small amount, e.g., an amount no greater than about 0.20mg/cm² ; and following which the so-processed innerlayers are utilizedin assembly of the multilayer structure.

The dissolution agent used for controllably dissolving and removing therequisite amount of the initially formed copper oxide conversion coatingcan be selected from a number of useful solutions having the capabilityof effecting the controlled dissolution without adversely affecting theoverall topography attained as a result of the copper oxide formation,including dilute solutions of mineral acids such as sulfuric acid;solutions of weak acids, such as organic acids or their salts, includingsubstituted or unsubstituted monocarboxylic acids, dicarboxylic acidsand tricarboxylic acids; and solutions of species capable of dissolvingcopper ions through complexation.

Another aspect of the present invention involves the recycle use ofdissolution agents, after having been employed to dissolve copper oxidefrom a number of innerlayers processed according to the invention (andthus containing a quantity of dissolved copper ions therein), toformulate electroless copper plating baths which can then be used in anyprocess where electroless copper plating is desired. This aspect of theinvention provides a ready and unexpected outlet for what wouldotherwise be a waste solution which would normally require processingtreatment in order to remove and recover copper values therefrom beforere-use as a dissolution agent and/or yet further processing as a wastestream. As a result, the overall process becomes even more economical.

DETAILED DESCRIPTION OF THE INVENTION

The generalized procedures for fabricating a multilayer printed circuitare quite well known in the art.

The circuitry innerlayers of a multilayer circuit are generallypatterned in a desired circuitry configuration, although use also can bemade of one or more inner-layers which are essentially continuous metallayers for use, e.g., as ground or power planes or the like. Forpatterned circuitry innerlayers, each is itself a printed circuit,consisting of copper circuitry on one or both sides of an insulatingsubstrate material. In the typical fabrication of a patterned circuitryinnerlayer, the starting material is a copper foil-clad dielectricsubstrate material, which can be any suitable thermosetting orthermoplastic resinous material such as epoxy, polyimide, polyester,phenolic, polyethylene, fluorocarbon polymer, co-polymers thereof, andthe like, generally with inorganic material reinforcement such as glassfibers or the like.

For patterned circuitry innerlayers, the pattern is arrived at, on oneor both sides of the substrate, by conventional techniques whereby anetch-resistant material is arranged on the copper foil surface in thepositive of the pattern of eventually desired circuitry. The resist,generally an organic resinous material, can be arranged in the desiredpattern by any number of means, but the predominantly employed processis through use of a photoresist layer which is then imagewise exposedand developed to leave the desired resist pattern. Thereafter, theinnerlayer is subjected to the action of a copper etchant, whereby allexposed copper (i.e., that not protected by the resist) is etched awaydown to the substrate surface. When the resist is thereafter removed,the innerlayer surface then has copper foil in the desired circuitrypattern.

In accordance with the present invention, as described in further detailhereinafter, the copper foil circuitry pattern so-provided would then beprocessed to form on its surfaces, and then controllably dissolve away asubstantial portion of, a copper oxide conversion coating, beforeutilizing the innerlayer in assembly of the multilayer circuit.

It is not strictly necessary that the copper circuitry of patternedcircuitry innerlayers consist of the copper foil of the original copperfoil cladding. It is also possible, for example, to additively producethe circuitry of the innerlayers, such as by starting with a baresubstrate material, patterning a plating resist thereon in the negativeof the circuitry pattern, and then electrolessly depositing copper onthose areas not covered by the resist. Another variation is to employ acombination of additive and subtractive processes in forming theinnerlayer circuitry. This variation is particularly useful in providinginnerlayer circuitry having a substantial thickness of copper, and evenmore particularly useful in fabricating a multilayer printed circuithaving buried through-holes, i.e., through-holes (or "vias")conductively interconnecting particular layers of innerlayer circuitrybut not common to the through-holes later formed through the entirety ofthe multilayer composite. In this latter regard, a double-sided copperfoil-clad substrate has through-holes drilled therein which are thenmetallized via electroless copper depositing (at the same time providingelectroless copper over the copper foil cladding). Typically theelectroless copper surface will then be patterned with a plating resistin the negative of the desired pattern of circuitry and the exposedareas then further built up with metal by electroless or electrolyticmethods. The built up areas are then protected by an etch resist, theplating resist removed, the electroless copper and copper foilthereunder then etched away down to the substrate surface, and the etchresist then removed to provide the desired pattern of built up coppercircuitry and plated through holes. The circuitry innerlayer so producedis then processed according to the invention for use thereafter inassembly of the multilayer structure.

As such, the inventive formation, followed by controlled dissolution, ofa copper oxide conversion coating, can be employed on a copper surfaceconsisting of copper foil or electroless copper or electrolytic copperdepending upon how the innerlayer circuit was produced. Also, whenreference is made herein to "copper", it should be understood that theterm embraces not only substantially pure copper but also suitablecopper alloys or intermetallics useful in printed circuitry.

Once the circuitry innerlayer is formed, and irrespective of whether itsouter-facing metallic copper circuitry surface is copper foil orelectroless or electrolytic copper, the method of the present inventionis then employed to further prepare the innerlayer for use in assemblyof the multilayer laminate.

The first step in this process is to form on the metallic coppersurfaces a copper oxide conversion coating.

It is possible to provide the requisite copper oxide coating by anynumber of means (including controlled air oxidation), but by far themost preferred route is by surface oxidation using a copper oxidizingsolution. As previously noted, there are a great many known solutionsand processes for providing this copper oxide conversion coating, withthe very substantial majority being based upon combination of alkalimetal or alkaline earth metal chlorite and alkali metal hydroxide in avariety of concentrations and for use in a variety of processingconditions. See, e.g., Landau, U.S. Pat. Nos. 4,409,037 and 4,844,981,and the references cited therein, such as U.S. Pat. Nos. 2,364,993;2,460,896; 2,460,898; 2,481,854; 2,955,974; 3,177,103; 3,198,672;3,240,662; 3,374,129; and 3,481,777, as well as U.S. Pat, No. 4,512,818,all of which are expressly incorporated herein by reference. The artvariously refers to such solutions and processes in terms of the colorand/or thickness of the copper oxide layer so-produced, e.g., thickblack oxide, thin black oxide, brown oxide, bronze oxide, red oxide, andthe like.

It is appropriate to again note here that the bonding surface of thecircuitry innerlayer according to the present invention is not copperoxide per se, but rather is believed to involve the underlying metalliccopper surfaces--whose topography has been altered by the controlledformation thereon of the copper oxide conversion coating followed bysubstantial dissolution of the so-formed oxide therefrom in a mannerwhich does not adversely affect the desirably attained topography--aidedby the small amount of copper oxide which is arranged to be present onthese metallic copper surfaces at the time of innerlayer and pre-pregassembly into a multilayer structure. In the art, the choice of anyparticular means for forming a particular oxide has primary reference tothe capability of the so-formed oxide itself to promote suitableadhesion of the circuitry innerlayer to the cured pre-preg dielectricsubstrate layer in contact therewith. As such, the oxide choice and thenecessary thickness thereof may vary depending upon the type ofdielectric material used as pre-preg layers (e.g., epoxy, polyimide,etc.), and other factors, and can be somewhat limited. In contrast, theearlier-noted bonding mechanism involved in the present invention issuch as to permit considerably wider latitude in the particular copperoxide, processes and thicknesses suitable for the initial step of copperoxide formation. Thus, rather than having reference to adhesionpromotion capability of the oxide per se, the choice of any particularoxide and process is based upon the functional requirements that information of the oxide there is achieved a suitable topographicalteration of the metallic copper surface, and that the oxide is onecapable of controlled removal under conditions which will not adverselyaffect the so-generated metallic copper surface topography, such thatthe metallic copper surface, with a small amount of copper oxidethereon, enters into a suitably sound bond with pre-preg layers incontact therewith upon assembly and curing of the multilayer structure.

The most preferred solutions and processes among this wide choice aresolutions based upon sodium or potassium chlorite and sodium orpotassium hydroxide, and using processes (immersion or spraying)involving temperatures preferably less than about 200° F., morepreferably less than about 160° F. Within these ranges, and dependingupon the particular concentrations and conditions, there can be producedcopper oxide conversion coatings on the metallic copper circuitrysurfaces of a variety of thicknesses, colors and compositions. Generallyspeaking, the thickness of the copper oxide conversion coating to beattained is functionally dictated by that which results from the extentof treatment needed to obtain the desired topography of the underlyingmetallic copper, and in general is essentially that known in the art tobe useful in conventional situations wherein copper oxide is to beutilized as the primary promoter of adherent bonding. As a generalmatter, the copper oxide conversion coating will typically provide acoating thickness of at least about 0.05 mg/cm², and generally betweenabout 0.05 and 0.6 mg/cm².

For forming the oxide coating, the metallic copper circuitry surfacestypically will first be cleaned to remove contaminants therefrom,including a micro-etching of the surface such as with aperoxide/sulfuric acid etching solution.

Following the formation of the copper oxide conversion coating on themetallic copper surfaces of the circuitry innerlayer, the formed oxideis then deliberately and controllably dissolved to the requisite extentby direct dissolution in a suitable dissolution agent. It is to be keptin mind that this step is essential no matter what the initial amount ofcopper oxide, i.e., whatever amount is provided in the initial copperoxide formation step, there must follow the controlled dissolution stepwhich decreases that initially formed amount.

In the dissolution of the previously formed copper oxide conversioncoating, the primary functional requirements are that the dissolution beconducted in a manner which does not substantially adversely affect thedesirable copper surface topography attained in the copper oxideformation step, and that the dissolution be carried out to an extentsuch that, with reference to the time when the circuitry innerlayer isarranged in contact with the pre-preg layer for forming the multilayerassembly, copper oxide exists on the metallic copper surfaces but onlyin a relatively very thin layer, e.g., in an amount on the order of lessthan about 0.20 mg/cm², and, in any event, an amount less than thatinitially provided.

Given these requirements, it is found that the preferred dissolutionagents are those which exhibit the capability of dissolving copperoxide, but which are not so aggressive in this regard (by their natureper se or the conditions at which they must be employed) as to precludethe ability in practice to control the dissolution to whatever extentdesired. Yet further, the dissolution agent must be one which isfunctional to achieve this controllable dissolution in a manner whichdoes not at the same time bring about any substantial loss of thetopography already attained in the underlying metallic copper surface.

In these regards, the preferred dissolution agents will include diluteaqueous solutions of otherwise strong mineral acids, such as dilutesolutions of sulfuric acid, and solutions of inherently weak acids (orsalts thereof), typically organic acids such as monocarboxylic acids(e.g., acetic acid, propionic acid, acrylic acid), dicarboxylic acids(particularly those having an odd number of carbon atoms in thebackbone) (e.g., malonic acid, glutaric acid), and tricarboxylic acids(e.g., citric acid), whether unsubstituted or substituted with halo-,amino-, hydroxyl- or methylene- or other like groups (e.g.,trichloracetic acid, glycine, malic acid, tartaric acid, glycolic acid,itaconic acid). Also useful are species which effect dissolution throughcomplexation with ionic copper, including nonionic species such assubstituted hydroxyalkyl alkyl amines (e.g., tetra (2-hydroxypropyl)ethylenediamine); ionic species such as alkyl carboxylic acid alkylamines (e.g., ethylenediamine tetraacetic acid, hydroxyethylethylenediamine triaacetic acid); and salts and mixtures of theforegoing species.

The particular conditions and time of contact of the copper oxideconversion coating with the dissolution agent will, of course, varydepending upon the particular dissolution agent employed and the extentof copper oxide dissolution necessary. Conditions suitable forparticular agents are illustrated in the Examples presented hereinafter.

As earlier noted, the dissolution procedure is carried out for a timeeffective to controllably remove sufficient copper oxide such that, withreference to the time when the circuitry innerlayer is arranged incontact with a pre-preg layer in assembling the multilayer structure,copper oxide exists on the copper surfaces of the circuitry innerlayerbut in an amount which is less than that initially provided, and whichin absolute terms is a relatively small amount. Preferably, this smallamount of copper oxide is no greater than about 0.20 mg/cm² ; morepreferably, the amount is no greater than about 0.12 mg/cm² ; yet morepreferably, the amount is no greater than about 0.08 mg/cm² ; and stillmore preferably, an amount no greater than about 0.05 mg/cm².

In the most preferred embodiment of the invention, the dissolutionprocess will be suitably controlled so as to leave behind on themetallic copper surfaces a thin layer of the earlier-formed copperoxide, in the mg/cm² amounts earlier described. It is also found,however, that the aims of the invention are substantially attained if infact the dissolution process effects removal of essentially all theoriginally-formed copper oxide, inasmuch as the eventually requiredpresence of a small amount of copper oxide on the metallic coppersurfaces of the circuitry innerlayer at the time when the innerlayer isarranged in contact with a pre-preg layer can be arranged to form on thecopper surfaces as a result of air oxidation and baking steps in theinterim between the dissolution procedure and the assembly of themultilayer structure. This so-formed oxide, in similar manner to thesituation where the oxide is present as the remains, after thedissolution step, of the initially-formed copper oxide conversioncoating, serves to aid in the bonding of the topographically alteredcopper surface to the pre-preg layer, and yet, either by reason of itssmall amount and/or the manner in which it is obtained, or otherfactors, exhibits decreased tendency, as compared to conventional oxideprocesses, for the local dissolution/delamination situation insubsequent through-hole processing.

With the innerlayers processed according to the invention, there is thenassembled the multilayer structure. In the assembly, an alternatingarrangement of circuitry innerlayer, one or more pre-preg layers,circuitry innerlayer, one or more pre-preg layers, etc. is provided inwhatever number and sequence appropriate for the eventually desiredmultilayer circuit. Indeed, as the number of innerlayers increases sotoo does the advantage of the present invention, inasmuch as the numberof areas for possible delamination also correspondingly increases. Thedielectric material used for the pre-preg layers can be any suitableheat and pressure curable resin, and typically the pre-preg will consistof a reinforcing material (e.g., glass cloth) impregnated with thecurable resin (epoxy, polyimide, or the like).

The multilayer structure is converted to an integral cured and bondedcomposite by subjecting it to heat and pressure for an appropriate time.Typical laminating conditions are pressures in the range of 300 to about400 psi, at a temperature of about 150° C. to about 205° C., for a timeof from about 1 to 4 hours. Following lamination, there often will be apost-baking of the laminate (e.g., at about 150° C.).

In the further processing of the multilayer composite to form amultilayer printed circuit, a number of through-holes will be drilledthrough the composite and the through-holes then metallized.

The processes for metallizing through-holes of multilayer printedcircuits are well known in the art and do not require extensivediscussion. Typically, the through-hole surfaces will first be desmearedto remove resin smear from the innerlayer edges exposed at thethrough-hole (e.g., via sulfuric acid, plasma desmearing, alkalinepermanganate, etc.), and may still be further treated to prepare thethrough-hole surfaces for full and adherent receipt of electrolessmetal, such as by post-desmear processing, etch-back, or the like. Thesurfaces of the through-hole are then further prepared for metallizationthrough use of conditioning and catalyzing steps, followed bymetallization using an electroless copper plating bath.

A complete description of through-hole metallization processes, andparticular steps and compositions for use therein, can be found in avariety of patents and publications. Useful reference can be had toKukanskis et al, published PCT application WO 89/10431, and thereferences cited therein, all incorporated herein by reference.

In the course of the through-hole metallization process, the outerlayersof the multilayer composite will also be processed to provide circuitrypatterns thereon. For example, the electroless metallization of thethrough-holes also provides electroless metal over the outerlayersurfaces. The surfaces can then be patterned with plating resist in thenegative of the ultimately desired circuitry, followed by build up ofthe exposed copper areas (including through-holes) with additionalelectroless or electrolytic copper. The built up areas are thenprotected by an etch resistant material (e.g., electroplated tin-lead ororganic resin), the plating resist removed, and the copper previouslythereunder then etched away down to the substrate surface. Thereafter,the multilayer printed circuit can be further processed as necessary,and as well known in the art, to solder mask particular portions,preserve solderability of other portions, etc.

By virtue of the present invention, strong adherence between circuitryinnerlayers and cured pre-preg layers is obtained. As such, oneadvantage is in the bonded strength of the overall laminate, such thatit is resistant to delamination which might otherwise be caused by thestresses of hole drilling and other severe processing steps, as well asin conditions of use. Another significant advantage is that since thecopper surface of the circuitry innerlayer contains only a small, yetimportantly present, amount of copper oxide, and perhaps also by reasonof the manner in which obtained, the surface exhibits a decreased levelof chemical alteration during through-hole metallization and othersubsequent processing, as compared with conventional copper oxidepromoted adhesion employing relatively large amounts of directly formedcopper oxide. Thus, the often stringent conditions of such processingdoes not lead to any significant localized chemical change in the bondedsurfaces and resultant localized delamination. Of particular importanceis that the foregoing result is obtained without counter-productivecompromise of the adhesion per se of the entire multilayer circuit.

The invention is further described and illustrated with reference to thefollowing examples.

EXAMPLE I

An epoxy-glass laminate clad on both sides with copper foil, and acorrespondingly-sized piece of 2 oz. copper foil, were both subjected(after scrubbing of the laminate) to the following process steps toproduce on their copper surfaces a substantially uniform copper oxideconversion coating having a thickness of about 0.5 mg/cm².

(a) cleaning in a commercial cleaner (OmniClean CI; MacDermid, Inc.,Waterbury, Conn.) for 5 minutes at 150° F.;

(b) water rinse;

(c) microetching in an ammonia-free commercial microetchant solution(Microetch G-4, one pound per gallon; MacDermid, Inc.) at 90° F. for 3minutes;

(d) water rinse;

(e) immersion in 10% sulfuric acid (room temperature) for 2 minutes;

(f) water rinsing;

(g) immersion in a commercial conversion oxide-forming solutioncontaining sodium chlorite and sodium hydroxide (MaCuBlack LT 9281, 100%activity in caustic; MacDermid, Inc.) for 5 minutes at 160° F.;

(h) water rinsing;

(i) water rinsing.

The laminate and foil were then each immersed in 0.5% sulfuric acid atroom temperature for one minute to effect substantial dissolution of thecopper oxide conversion coating, followed by rinsing and drying. Thefoil surfaces at this stage had a slightly non-uniform matte dark pinkappearance.

The laminate and foil each were baked at 135° C. for 30 minutes. Theso-processed copper clad laminate and copper foil, having less thanabout 0.2 mg/cm² copper oxide on their copper surfaces, were thensandwiched about a suitably-sized B-stage pre-preg layer (glass clothimpregnated with epoxy resin) and the composite then laminated at350-400 psi and 160° C. for one hour. After patterning and alkalineetching of the outer-facing copper foil piece to provide a circuitrypattern, the adhesion of the copper foil to the cured B-stage pre-pregin the multilayer composite was tested and found to be about 6.5 to 7.0lbs/inch.

A sample of this multilayer composite was immersed in 10% hydrochloricacid at 110° F. for 30 minutes, and no evidence was seen of attack bythe acid between the copper foil and cured B-stage at the edges of thesample. In addition, a sample of the multilayer composite was drilled toprovide a number of through-holes therein, and the through-holes weremetallized in a process utilizing sulfuric acid etchback, solventswelling, alkaline permanganate desmear, glass etching, through-holeconditioner, electroless copper catalytic activation, electroless copperplating (20 microinches) and over-plating of electrolytic acid copper (2hours at 15 ASF). Following the metallization, no evidence was seen ofattack of the copper foil/cured B-stage interface, nor of localizeddelamination at or near through-hole surfaces.

EXAMPLE II (Comparison)

Example I was identically repeated with the exception that the immersionof laminate and foil in the 0.5% sulfuric acid solution after formationof the copper oxide conversion coating was eliminated. The adhesion ofthe copper foil to the cured B-stage pre-preg in the multilayercomposite was only about 3 lbs/inch. The immersion of a sample of themultilayer composite in 10% hydrochloric acid produced attack on theoxide for a depth of 15 mils from the sample edge. For the metallizedthrough-hole sample, pink rings extending 4 mils beyond the holeperimeter were observed.

EXAMPLE III

Example I was identically repeated with the exception that the immersionof the laminate and foil in the 0.5% sulfuric acid solution afterformation of the copper oxide conversion coating was replaced byimmersion for 5 minutes at 150° F. in a 30 g/l solution of malonic acid,adjusted to pH 2.5 with sodium hydroxide, resulting in a uniform, matte,dark pink appearance on foil surfaces. The adhesion between foil andcured B-stage pre-preg in the multilayer composite was about 7 lbs/inch,and the same results as in Example I were achieved upon immersion in 10%hydrochloric acid and upon through-hole metallization.

EXAMPLE IV (Comparison)

Example I was identically repeated with the exception that the immersionof the laminate and foil in the 0.5% sulfuric acid solution afterformation of the copper oxide conversion coating was replaced byimmersion in 0.18N phosphoric acid. The adhesion between foil and curedpre-preg in the multilayer composite was only 1.2 lbs/inch, andtreatment of a sample with the 10% hydrochloric acid resulted in attackof the foil/cured pre-preg interface to a depth of about one mil.

EXAMPLE V (Comparison)

Example I was identically repeated with the exception that the immersionof the laminate and foil in the 0.5% sulfuric acid solution afterformation of the copper oxide conversion coating was replaced byimmersion in 10% sulfuric acid. The foil surfaces had a red-pinkappearance, and the adhesion between foil and cured pre-preg in themultilayer composite was only 3.2 lbs/inch.

EXAMPLE VI

Example I was repeated with the exception that the copper oxide formingtreatment of step (g) utilized a different commercial formulation,though similarly containing alkali metal chlorite and alkali metalhydroxide (OmniBond Oxide, brown formulation; MacDermid, Inc.), and withthe further exception that the immersion of the laminate and foil in the0.5% sulfuric acid solution after formation of the copper oxideconversion coating was replaced by immersion for 5 minutes at 140° F. in0.18M citric acid (adjusted to pH 4.0 with dilute sodium hydroxide). Theadhesion between foil and cured pre-preg in the multilayer composite wasabout 6-7 lbs/inch, and no attack was evident in the 10% hydrochloricacid treatment test.

EXAMPLE VII (Comparison)

Example VI was identically repeated but with elimination of the citricacid treatment. Adhesion values of 13-14 lbs/inch were obtained, buttreatment in the 10% hydrochloric acid resulted in attack of the oxideto depths ranging from 2 to 10 mils from the sample edges.

EXAMPLE VIII

Example I was identically repeated with the exception that the copperoxide forming treatment of step (g) utilized a different commercialformulation, though similarly containing alkali metal chlorite andalkali metal hydroxide (OmniBond Oxide, thick black formulation;MacDermid, Inc.) to produce an oxide layer of 0.58 mg/cm², and with thefurther exception that the immersion of the laminate and foil in the0.5% sulfuric acid solution after formation of the copper oxide coatingwas replaced by immersion for 5 minutes at 120° F. in a 50 g/l solutionof malic acid (pH 1.9), producing on the foils a uniform brown surface(about 0.05 mg/cm² oxide). The adhesion between foil and cured pre-pregin the multi-layer composite was 10.5 lbs/inch, and no evidence ofattack in the 10% hydrochloric acid treatment test, or followingthrough-hole metallizing, was apparent.

The foregoing Examples demonstrate the unique features and requiredcooperating interactions involved in the present invention.

Fundamentally, the first requirement is that the copper surfaces of thecircuitry innerlayers be provided with a conversion coating of copperoxide. The copper oxide coating is not in and of itself relied upon asthe primary bonding surface for innerlayer/pre-preg bonding as inconventional processes, but instead is necessary as a means to an end,i.e., its formation promotes the development in the underlying metalliccopper surface of a topography which enables the copper to adherentlybond to a cured pre-preg layer.

Given this consideration, another required element of the invention isthe deliberate removal of a significant portion of the so-formed oxide,thus substantially revealing the underlying topographically desirablebonding surface and, importantly, serving to substantially decrease theoxide dissolution which otherwise would occur in subsequent processingof the multilayer composite and the consequent localized delaminationtendency caused by this dissolution of a primary bonding surface.

As the Examples also demonstrate, this dissolution of the initiallyformed oxide is not simply a matter of just getting rid of most of theoxide; instead, the process must be carried out in a controlled mannerwhich does not adversely alter the underlying topography already inplace.

In terms of the requisite extent of oxide removal, the criteria areessentially functional. At a minimum, of course, it is necessary toremove at least an amount sufficient to decrease the importance of oxideas a bonding surface for the pre-preg, since otherwise the localizedoxide dissolution which later would occur in through-hole processingwill run the risk of producing localized delamination. It is for thisreason that the present invention seeks to provide a copper surface, atthe time of innerlayer/pre-preg assembly, having no greater thanspecified amounts of copper oxide thereon. Nevertheless, it presentlyappears necessary that, however small, some amount of copper oxide mustbe present on the copper surface at the time of innerlayer/pre-pregassembly, whether by its remainder after the dissolution step or itsre-formation in air after complete removal in the dissolution step.Still further, the amount provided must have been arrived at by theinitial formation of copper oxide, followed by controlled dissolution,no matter how little oxide per se might be present after initialformation, i.e., even if it was initially provided in an amount alreadyat or below the amounts heretofore discussed (e.g., no more than about0.20 mg/cm², etc.) with reference to the oxide present at the time oflamination of innerlayer and pre-preg.

In connection with the present invention, a unique process also has beendiscovered for effecting the treatment of the copper oxide-coated coppersurfaces with the dissolution agent for controllable dissolution andremoval of copper oxide. The process is particularly suitable for thosecommercial scale operations in which a significant quantity ofinnerlayers is processed.

As previously noted, the preferred method for carrying out the inventionis by immersion of the inner-layers, whose copper surfaces have aconversion coating of copper oxide thereon, in a dissolution agent for atime and at conditions which will effect the requisite degree ofcontrollable dissolution and removal of the copper oxide. For commercialoperations, it is most preferred that the requisite degree of suchdissolution/removal be arranged to be a function simply of immersioncontact time. In other words, the most desirable and simple form ofprocess control is the pre-establishing, for a given type and thicknessof copper oxide coating, a given dissolution agent, and a givenoperating temperature, of a set immersion time (or narrow range thereof)which essentially invariably produces the requisite extent of oxidedissolution. The problem arises, however, that process control in thismanner is highly complex in this process because the dissolution agentbecomes progressively more laden with copper ions and thus becomesprogressively less efficient in copper oxide dissolution.

Thus, as immersion processing progresses, any pre-determined immersiontime for achieving the requisite degree of oxide dissolution based upon,e.g., a freshly made-up solution of dissolution agent, will be foundover time not to be effective in attaining the desired degree ofdissolution. While it may be possible to alleviate this problem withfrequent replacement of dissolution agent with freshly made-up solutionsthereof, processing in this manner is economically disadvantageous.

According to the invention, the immersion contact of the copperoxide-coated innerlayers with dissolution agent is carried out in aprocess in which the dissolution agent is substantially continuouslycirculated from the innerlayer contact zone (e.g., immersion vessel orportion thereof), into contact with an ion exchange resin, and thenreturned to the innerlayer contact zone. The ion exchange resin, whichtypically is housed in a cylindrical or other suitably-shaped cartridge,will be composed of any suitable ion exchange resin effective to removecopper ions from the particular dissolution agent solution by ionexchange mechanism, thereby simultaneously regenerating the dissolutionagent solution to its initial form. Given the acidic nature of the mostpreferred dissolution agent solutions utilized in accordance with thepresent invention, the preferred ion exchange resin will be a cationexchange resin, particularly of the strong acid type such as is the casefor sulfonated resins. Thus a typical and preferred resin will comprisea suitable resin foundation, such as polystyrene cross-linked withdivinyl benzene, which is then sulfonated to provide --SO₃ H groups foreffecting the ion exchange function. Although less preferred,utilization of weak acid forms of cation exchange resins, such as thosebased upon pendant --COOH groups, also is possible for certaindissolution agent solutions. In either case, of course, the particularresin is chosen on the basis of the particular nature of the dissolutionagent to be processed thereby, i.e., such that the pendant exchangegroups of the resin will be sufficiently dissociated in the environmentof the solution being treated such that exchange of hydrogen ion forcopper ion in the dissolution agent solution is brought about.

As is well known in the art, ion exchange resins are available in a widevariety of bead sizes and porosity, and can be arranged to provide awide range of volumetric capacities. For any given type of dissolutionagent solution in question, and known or expected copper ionconcentration therein after processing of innerlayers according to theinvention, and for any given throughput of dissolution agent solutiondesired for the overall process, it is well within the skill of the artto select an exchange resin of suitable physical properties forachieving the required degree of removal of copper ions from thedissolution agent solution such that overall process control based uponcontact time of innerlayers with dissolution agent is possible. While itis possible to operate, and establish times for, the overall immersionprocess using dissolution agents of any particular and substantiallyconstant concentration of copper ions, it is of course preferred thatthe dissolution of oxide be effected with a dissolution agent having thelowest possible copper ion concentration so as to attain the mostefficient and rapid dissolution possible. As such, the resin type,volume and volumetric throughput will preferably be chosen such that theconcentration of copper ion in the dissolution agent after passagethrough the ion exchange resin will be on the order of less than about0.5 g/l, and most preferably less than about 0.1 g/l. With substantiallycontinuous recirculation and ion exchange treatment, such concentrationscan be substantially constantly maintained throughout innerlayerprocessing.

As the ion exchange resin itself becomes laden with copper ion, it canbe regenerated to elute the copper ions therefrom and restore the resinto its original form for further ion exchange. As an additional benefit,copper values are recovered in an efficient and cost-effective manner.

The foregoing process is illustrated in the following example.

EXAMPLE IX

A series of panels of copper-clad epoxy-glass laminate were identicallytreated to provide on the copper surfaces a thick black oxide conversioncoating. A bath was prepared containing 50 g/l of malic acid (pH about1.9) at about 120° F. for effecting controlled dissolution and removalof copper oxide from the panel surfaces, while substantially maintainingthe topography of the copper surfaces, to an extent such that the panelsurfaces would have approximately 0.05 mg/cm², of oxide thereon at theconclusion of the treatment.

For effecting the dissolution, the bath (total volume, 13 gallons) wascontinuously circulated from the immersion processing tank, to andthrough an ion exchange cartridge, and returned to the processing tank,at a rate of about 10 liters per minute. The ion exchange cartridgehoused approximately 0.6 cubic feet of a cation exchange resin in strongacid form, consisting of polystyrene cross-linked with divinyl benzeneand sulfonated to provide --SO₃ H groups (Sybron Ionac C-267 ionexchange resin; available from Sybron Chemicals, Inc., Birmingham,N.J.).

Copper oxide-coated panels were immersed in the bath for 5 minutes, andthe processing of panels continued until about 1580 square feet ofcopper surface had been processed by the bath. Periodic sampling of thebath indicated a relatively constant copper concentration therein ofless than about 0.2 g/l over the course of processing, and a relativelyconstant pH of about 1.8 to 2.0. At the end of the processing run, itwas determined that the malic acid concentration of the bath declined byonly about 11%. All panels processed through the bath were substantiallyuniform in appearance and properties (peel strength, delamination). Inlamination tests, involving immersion of the laminate in 10%hydrochloric acid, only minimal evidence of pink ring was noted (<1mil), as compared to a 5 mil pink ring found when employing panels whichhad not been subjected to the malic acid treatment, and excellentadhesion values were obtained.

The ion exchange resin utilized in the foregoing process was regeneratedby thoroughly rinsing the resin with clean deionized water, followed bysequentially flowing four 5-gallon batches of 10-20% sulfuric acidthrough the resin (30 minute contact time for each batch), thus elutingcopper ion from the resin in the form of copper sulfate while at thesame time returning the resin to its acid (hydrogen ion) form.Associated filter and pump devices were then thoroughly rinsed withdeionized water until all rinse water tested negative for sulfate ion(absence of precipitate with barium ion). The regeneration recovered 505grams of copper, as compared to an estimated 589 grams of copperdissolved by the malic acid bath during the processing run, for arecovery yield of about 85%. The regenerated resin was utilized again inthe foregoing process sequence, with no difference in results.

In yet another aspect of the present invention, there is provided aprocess for recycle use of dissolution agents after they have beenemployed to dissolve copper oxide from a number of innerlayers processedaccording to the invention. This recycle use of copper-containingdissolution agent is especially applicable in those processes accordingto the invention in which a significant number of copper oxide coatedinnerlayers will be processed with the dissolution agent forcontrollably dissolving and removing copper oxide. It is most especiallyuseful for those processes in which use is not made of theearlier-described recirculation/ion-exchange process for removing copperions from the dissolution agent, and which, therefore, generate copperion-containing dissolution agent solutions which at some point are soladen with copper ion that they must be replaced by fresh solutions andwhich must otherwise be subjected to processing treatments to recovercopper values therefrom.

According to this aspect of the invention, the copper-containingdissolution agent solution which results from processing innerlayers tocontrollably dissolve copper oxide therefrom, is utilized in formulationof an electroless copper plating bath which can then be employed in anyand all known processes for the electroless copper plating of metallicor non-metallic substrate surfaces.

Electroless copper plating baths, i.e., baths for use in the plating ofcopper by chemical reduction as opposed to electrolytic means, arewell-known in the art and comprise aqueous solutions containing asessential ingredients a solution-soluble copper salt as a source ofcupric ions; reducing agent capable of reducing cupric ions to metalliccopper; complexing agent for maintaining copper ions in solution; and,if necessary, suitable agents for achieving the requisite solution pH.Other optional components include stabilizers, levelling agents, grainrefiners and the like, all as well-known in the art. A typical coppersalt for use in providing cupric ions is copper sulfate, and thereducing agent for the copper ions can be, e.g., formaldehyde, ahypophosphite or a borane. The choice of complexing agents will bedependent upon the particular reducing agent of the bath and the pH ofthe bath, but typically will be compounds such as ethylene diaminetetraacetic acid (EDTA) (or salts thereof) and similar such compoundsfor highly alkaline (pH≧11) formaldehyde-reduced baths. Forhypophosphite-reduced baths having a pH of from 5 to 11, suitablecomplexers will be EDTA, N-hydroxyethyl ethylenediamine triacetic acid,nitrilotriacetic acids, and salts thereof, and for hypophosphite-reducedbaths having a pH from 9 to 13 complexers such as tartaric acid andsoluble tartrates can be employed.

As earlier-indicated, electroless copper baths per se are well-known inthe art. Information regarding typical bath formulations and operatingconditions may be found, e.g., in the chapter entitled "ElectrolessPlating" at pp. 353-363 of the 1992 Metal Finishing Guidebook Directory,and in the text Electroless Plating, Fundamentals & Applications(Mallory & Hajdu, Editors, 1990), particularly at pp. 289-375, bothincorporated herein by reference.

In the process of the present invention, dissolution agents which areuseful in controllably dissolving the conversion coating of copper oxidefrom the copper surfaces of innerlayers are in many cases comprised ofcompounds which are useful as components of an electroless copperplating baths. Moreover, the nature of the controlled copper oxidedissolution is such that, upon treatment of a number of innerlayers, thedissolution agent will contain copper ions in solubilized and/orcomplexed form in a concentration not unlike that required in anelectroless copper plating bath. Yet further, the nature of the overallprocess, i.e., formation of a copper oxide conversion coating on acopper surface followed by controlled dissolution of the copper oxide,also is such as to produce a copper-containing dissolution agent whichis essentially free from contamination from other metal ions and thuseminently consistent with the metal purity requirements for anelectroless copper plating bath.

This aspect of the invention, then, in which dissolution agents whichhave been utilized in the controlled dissolution of copper oxide inaccordance with the process earlier described herein are employed informulation of an electroless copper plating bath, will be applicable inall those situations in which the dissolution agent is one which is notincompatible with the nature, mechanism and properties of an electrolesscopper plating bath. For example, where the dissolution agent is adilute solution of sulfuric acid (see EXAMPLE I herein), its compositionafter use in dissolving copper oxide will essentially be dilute sulfuricacid and dissolved copper sulfate; a solution of this type can beemployed to formulate an electroless copper bath by combining it withsuitable reducing agent, complexers and pH adjustors, as well asadditional cupric ion if necessary. In like manner, a dissolution agentcomprised of a mixture of dilute sulfuric acid and EDTA will, after itsuse in copper oxide dissolution, comprise a mixture of dilute sulfuricacid, EDTA and cupric ions as copper sulfate and/or already in complexwith EDTA. As such, this composition is well-suited for use informulating an electroless copper bath by combining it with a reducingagent (e.g., formaldehyde) and other suitable agents, often without needfor supplementation with additional complexing agent.

In contrast, certain dissolution agents which are useful in effectingthe controlled copper oxide dissolution of the principal method of thisinvention produce recycle streams whose composition may be unsuitablefor use in formulating an electroless copper plating bath. This willgenerally be the case, for example, when organic acids alone are used asthe dissolution agent and/or where a complexing agent suitable for usein dissolution of copper oxide is either of insufficient complexingability to serve as a complexer in an electroless copper bath in thepresence of a reducing agent, or is too tenacious a complexer for use inan electroless copper plating bath.

The greatest overall process economics will be realized in thosesituations in which the dissolution agent is one which provides arecycle stream containing not only cupric ions but also a complexingagent which is suited for an electroless copper plating bath. This willbe the case for the aforementioned dilute sulfuric acid/EDTA dissolutionagents, as well as for dissolution agents comprised of dilute sulfuricacid and other complexing agents such as tartaric acid, N-hydroxyethylethylenediamine triaacetic acid, diethylenetriamine pentaacetic acid andthe like (or salts or mixtures thereof). With these recycle streams itis often possible to formulate an electroless copper plating bathwithout need for overt addition of any further source of cupric ions orcomplexing agent, or at the very least with only minor amounts of suchadditions being required. In this way, highly economical electrolesscopper plating baths are arrived at, and can largely offset the costsassociated with the copper oxide dissolution step which is introducedinto the printed circuit fabrication process in accordance with thepresent invention.

The following examples illustrate this aspect of the invention.

EXAMPLE X

The process of Example I was utilized in providing innerlayers with aconversion coating of copper oxide followed by controlled dissolution ofthe so-formed copper oxide coating. The exceptions from the exactprocess of Example I were in the use of a different solution to form thecopper oxide conversion coating in step (g) (using instead of theMaCuBlack LT 9281, another sodium chlorite/sodium hydroxide formulationknown as OmniBond Plus, available from MacDermid, Inc.), and in the use,in place of the 0.5% sulfuric acid dissolution agent, a dissolutionagent comprising 15 g/l sulfuric acid and 50 g/l of the tetrasodium saltof EDTA, maintained at pH 3.3-4.0 and 130° F.

Epoxy-glass copper foil clad laminates were processed in the foregoingmanner until such time as the copper concentration in the sulfuricacid/EDTA dissolution agent had reached 0.36 Molar. At that time, 210 mlof the solution was utilized in formulating an electroless copperplating bath as follows:

    ______________________________________                                        Dissolution agent solution                                                                           210 ml                                                 Formaldehyde, 37%       20 ml                                                 Stabilizers            1.5 mg                                                 NaOH (Metex Electroless                                                                              100 ml                                                 Copper Reducer 9073E;                                                         MacDermid, Inc.)                                                              DI Water               To 1 Liter                                             ______________________________________                                    

The so-formulated bath had a pH of about 12.9.

A 3-inch×3-inch copper clad epoxy panel was stripped of its coppercladding, dried and weighed. The panel was then prepared and catalyzedfor electroless copper plating by contacting it sequentially (rinsesteps omitted) with a conditioner (MacDermid Conditioner 90, 5 minutes,120° F.), followed by an acid pre-dip (MacDermid 95 Predip, 2 minutes,room temperature), followed by activator (i.e., palladium/tin compounds,MacDermid Activator 95, 5 minutes, 90° F.), followed by accelerator(MacDermid Accelerator 97, 2 minutes, 125° F.). The panel was thenplaced in the above-described electroless copper plating bath and platedat room temperature with air agitation. Copper plating initiated rapidly(less than 10 seconds) and uniformly on the panel surface. After 20minutes, the panel was a uniform pink color and had a weight gain of0.049 grams.

After plating, the electroless bath (without air agitation) was allowedto stand covered for 24 hours at room temperature. At the end of thisperiod, no spontaneous decomposition of the bath had occurred,indicating that it was a suitably formulated stable bath.

EXAMPLE XI

The process of Example X was identically repeated in three additionaltests, with the exception that the dissolution agent employed tartaricacid, N-hydroxyethyl ethylenediamine triacetic acid, anddiethylenetriamine pentaacetic acid, respectively, in place of the EDTA.In each test, effective controlled copper oxide dissolution was obtainedand the electroless copper plating bath formulated from thecopper-containing dissolution agent was acceptable in all respects.

As will be apparent, then, this aspect of the invention involves atleast periodically taking at least a portion of the copperion-containing dissolution agent which has been used in the controlleddissolution of copper oxide from copper-clad dielectrics (and which thusbecomes increasingly more concentrated in copper ions over the course ofsuch dissolution treatment) and combining it with whatever componentsare necessary to arrive at a suitable electroless copper plating bath,i.e., a bath comprised of an aqueous solution of copper ions, a reducingagent capable of reducing copper ion to metallic copper, and acomplexing agent for maintaining copper ions in solution in the bathuntil plating with the bath occurs, as well as necessary pH adjusters,stabilizers, grain refiners, etc. The actual components which must becombined with the copper ion-containing dissolution agent so as toarrive at the desired electroless copper plating bath will of course bedependent upon the particular dissolution agent and the concentration ofmaterials therein at the time of combining other components with it tomake an electroless copper bath. In some cases, then, the copperion-containing dissolution agent will be useful solely as a source ofcopper ions, and thus will need to be combined with reducing agent,complexing agent and other optional components in order to arrive at auseful plating bath. In other cases, such as where the dissolution agentbegins as a composition containing a copper ion complexing agent, thecopper ion-containing dissolution agent which results from copper oxidedissolution may need to be combined only with reducing agent andoptional components in order to arrive at a useful electroless copperplating bath, i.e., with the requisite copper ions and copper ioncomplexing agent being already provided by the copper ion-containingdissolution agent. In yet other cases, the situation may be somethinginbetween, e.g., where the copper ion-containing dissolution agent whichresults from copper oxide dissolution provides some, but not all, of theconcentration of copper ions and complexing agent needed in theelectroless copper plating bath.

In all situations, however, process economics are realized by reason ofputting to beneficial use a process stream which otherwise would need tobe extensively processed to enable it to be either recycled for use as acopper oxide dissolution agent or waste treated.

It will be appreciated by those of skill in the art that the foregoingdescription and examples have been employed in illustration ofparticular features of the invention, and it is to be understood thatspecific compounds, amounts, process conditions, and the like are notintended as limitations upon the invention except as set forth in theappended claims.

What is claimed is:
 1. In a method for treating a copper circuitryinnerlayer prior to utilizing said innerlayer in assembly, with one ormore partially-cured dielectric substrate material layers, of amultilayer structure for a first multilayer printed circuit, in whichthe copper surfaces of said circuitry innerlayer are initially providedwith a conversion coating of copper oxide thereon to serve as anadhesion promoter, the improvement comprising:(a) immersing saidinnerlayer, having said conversion coating of copper oxide on itssurfaces, in contact with a dissolution agent effective to controllablydissolve and remove at least a portion of said copper oxide coating,whereby said dissolution agent becomes increasingly more concentrated indissolved copper ions; (b) at least periodically combining at least aportion of said dissolution agent containing copper ions with componentsnecessary to convert said dissolution agent containing copper ions to anelectroless copper plating bath which is suitable for use for depositingcopper on a surface of a second multilayer printed circuit, saidelectroless copper plating bath comprised of an aqueous solutioncomprising copper ions, a reducing agent capable of reducing said copperions to the metallic state, and a complexing agent for copper ions.
 2. Amethod according to claim 1 wherein said dissolution agent utilized forcontrollably dissolving and removing said copper oxide coating comprisesa dilute aqueous solution of sulfuric acid and a complexing agent forcopper.
 3. A method according to claim 2 wherein said complexing agentfor copper which is present in said dissolution agent is selected fromthe group consisting of ethylenediamine tetraacetic acid, N-hydroxyethylethylenediamine triacetic acid, diethylenetriamine pentaacetic acid,tartaric acid, salts thereof, and mixtures thereof.
 4. A methodaccording to claim 3 wherein the entire amount of complexing agent forcopper ions required in said electroless copper plating bath is providedby the complexing agent for copper present in said portion of saiddissolution agent containing copper ions.
 5. A method according to claim4 wherein said combining of said portion of said dissolution agentcontaining copper ions with components necessary to convert it to anelectroless copper plating bath comprises combining said portion withformaldehyde as a reducing agent for copper ions and with sufficientalkaline material to produce a bath pH of 11 or greater.
 6. A method forformulating an electroless copper plating bath, comprising:(a) providingthe copper surfaces of a copper-clad dielectric material with aconversion coating of copper oxide; (b) immersing said copper-claddielectric material, with said conversion coating of copper oxide on itscopper surfaces, in contact with a dissolution agent effective tocontrollably dissolve and remove at least a portion of said copper oxidecoating, whereby said dissolution agent comes to contain dissolvedcopper ions; (c) combining at least a portion of said copper ioncontaining dissolution agent with components effective to convert it toan electroless copper plating bath comprised of an aqueous solutioncomprising copper ions, a reducing agent capable of reducing copper ionsto metallic copper, and a complexing agent for maintaining copper ionsin solution in said bath.
 7. A method according to claim 6 wherein saiddissolution agent utilized for controllably dissolving and removing saidcopper oxide coating comprises a dilute aqueous solution of sulfuricacid and a complexing agent for copper.
 8. A method according to claim 7wherein said complexing agent for copper which is present in saiddissolution agent is selected from the group consisting ofethylenediamine tetraacetic acid, N-hydroxyethyl ethylenediaminetriacetic acid, diethylenetriamine pentaacetic acid, tartaric acid,salts thereof, and mixtures thereof.
 9. A method according to claim 8wherein the entire amount of complexing agent for copper ions requiredin said electroless copper plating bath is provided by the complexingagent for copper present in said portion of said dissolution agentcontaining copper ions.
 10. A method according to claim 9 wherein saidcombining of said portion of said dissolution agent containing copperions with components necessary to convert it to an electroless copperplating bath comprises combining said portion with formaldehyde as areducing agent for copper ions and with sufficient alkaline material toproduce a bath pH of 11 or greater.